Influenza virus attachment to host cells is mediated by a viral glycoprotein, the hemagglutinin, which binds to cell surface oligosaccharides containing sialic acid. The hemagglutinin can be quite specific for the sialyloligosaccharide sequence and the type of sialic acid to which it will bind. Furthermore, different hosts of influenza can apparently exert selective pressures resulting in the emergence of a receptor variant with properties optimal for growth in that host. It is a long term goal of this project is to understand at the molecular level how the interaction of influenza virus hemagglutinins with sialyloligosaccharide receptor determinants elaborated by the host influence the biology of these viruses. Specific goals of the proposed research are to assess recent influenza A (H1N1) isolates from a variety of species for their ability to bind sialyloligosaccharides of defined sequence. To this end, human erythrocytes that have been enzymatically modified by treatment with bacterial sialidases, to remove endogenous sialic acid, and then resialylated with one of the available mammalian sialyltransferases to contain a single sialyloligosaccharide of defined structure will be used as model membranes. The molecular basis for receptor specificity will be examined in detail by selecting receptor variants of influenza A (H1N1) isolates on the basis of their ability to bind specific sialyloligosaccharide sequences. The biological implications of receptor specificity for the influenza viruses will be explored by comparing the receptor binding properties of influeza A (H1N1) and influena A (H3N2) isolates adapted to growth in eggs and in mammalian cell cultures. The molecular mechanisms involved in adaptation of these viruses from mammalian cell culture to growth in eggs will be examined as a laboratory model of host adaptation in nature. Epidemic and non-epidemic isolates of influenza A (h3N2) viruses will also be compared for their relative affinities for cell surface sialyloligsaccharides to assess the importance of receptor binding in the epidemiologic behavior or these viruses. The results obtained should be useful in helping to understand the molecular basis and biological consequences of variation in the receptor specificity of the influenza virus hemagglutinin.